The role of subsurface currents in supplying the seasonal upwelling in the Eastern Equatorial Atlantic.

Upwelling is of great importance for the surface heat budget and primary productivity in the eastern equatorial Atlantic and the Gulf of Guinea. However, quantifying the upwelling transport, especially on a seasonal time scale, has remained a significant challenge. In this work we produce estimates of seasonal upwelling in two regions of the equatorial Atlantic, located between 23°-10°W and 10°W-0°E. Upwelling is estimated using two methods: from surface drifter divergence, and from estimates of subsurface mass divergence derived from Argo and equatorial current meter data. Mooring observations between 23°W, 10°W and 0 °E are used to derive the zonal component of the subsurface horizontal mass divergence and show how much the subsurface zonal currents, and most importantly the Equatorial Undercurrent (EUC), play a role in supplying the seasonal upwelling. The meridional component of the horizontal mass divergence is estimated applying equatorial geostrophy to obtain meridional currents from Argo T/S profiles. In both estimates the upwelling has a maximum value of ~8Sv between 23°-10°W and ~6Sv between 10°W-0°E. We find that between 23°W and 10°W, the EUC does not appear to play a significant role in supplying the upwelling in either of the estimates during late boreal spring and summer, when the upwelling is at a maximum. It is the geostrophic meridional flow that supplies the upwelling at this time, while the opposite is true during fall-winter with the EUC mainly supplying the upwelling. Between 10°W and 0°E, the EUC exhibits a much larger mass convergence, especially during boreal fall, that is however compensated through strong meridional geostrophic divergence and leads to weak upwelling at that time. The methods we apply to calculate the upwelling estimates are further tested using the output of a high resolution regional model.